Pressure sensor elements can be utilized to provide an indication of ambient pressure in an environment or to provide a pressure measurement between two adjacent or contacting surfaces. Sensor arrays can also provide a profile of such pressure over a given area, including variations in such profile with time. Uses for such arrays include providing a dental occlusion profile for fitting crowns, bridges or false teeth, providing a foot pressure profile for fitting shoes or for orthopedic purposes, providing a pressure profile at a gasket to assure proper sealing at a joint and in numerous other medical, industrial, military and other applications.
One form of such pressure sensors utilizes a first substrate having at least one first conductor formed thereon, a second substrate having at least one second conductor formed thereon, and a pressure sensitive coating over the conductors on at least one of the substrates, the substrates being mounted such that the conductors on one substrate intersect conductors on the other substrate, with the variable resistant material therebetween, a pressure point being formed at each such intersection. Examples of such pressure sensitive arrays and of control circuitry for use therewith are shown in U.S. Pat. Nos. 4,856,993 issued Aug. 15, 1989, 5,033,291 issued Jul. 23, 1991 and 5,505,072 issued Apr. 9, 1996. In some applications, there may be only a single conductor on each substrate with only a single sensor point being formed. Such sensors are sometimes also referred to as "button sensors."
Sensors of the type indicated above have found wide application and provide significant advantages. However, such sensors also have certain limitation. First, since atmospheric pressure exists within such sensors, they are normally not capable of measuring pressures below atmospheric pressure (i.e. 14.7 psi) and thus have a relatively large force threshold.
Second, these sensors are subject to "drift" or "creep" when under load for a period of time. In particular, with a constant load, such increase tends to slow down over time, so that the longer a sensor is under load, the less drift occurs per unit of time. This drift can impact the reliability of readings.
Such sensors may also be subject to a hysteresis which results in the force/output profile being different when load is being increased than when load is being reduced. This can result in some inaccuracy and non-repeatability in readings which is undesirable. Repeatability can also be adversely affected when the pressure sensitive layers move with respect to one another between loadings of the sensor.
A need therefore exists for an improved sensor which overcomes the various problems indicated above.